Many natural and synthetic gases comprise a variety of different hydrocarbons, and numerous separation processes and configurations are known in the art to produce commercially relevant fractions from such gases. In a typical gas separation process, a feed gas stream under pressure is cooled by a heat exchanger, typically using propane refrigeration when the feed gas is rich (containing more than 5% C3+ components) and as the gas cools, liquids condense from the cooled gas. The liquids are then expanded and fractionated in a distillation column (e.g., de-deethanizer or demethanizer) to separate residual components such as methane, nitrogen and other volatile gases as overhead vapor from the desired C2, C3 and heavier components.
For example, Rambo et al. describe in U.S. Pat. No. 5,890,378 a system in which the absorber is refluxed, in which the deethanizer condenser provides the reflux for both the absorber and the deethanizer while the cooling requirements are met using a turbo expander and propane refrigeration. Here, the absorber and the deethanizer operate at substantially the same pressure. Although Rambo's configuration advantageously reduces capital cost for equipment associated with providing reflux for the absorption section and the de-deethanizer, high ethane recovery of 80% becomes difficult when feed gas pressure is less than 1,000 psig due to lower turbo expansion cooling while reducing absorber pressure. Moreover, where the gas has a high CO2 content (e.g., greater than 2 mole %) expansion cooling is problematic due to the potential of CO2 freezing in the demethanizer. Consequently, such plants typically require deep propane refrigeration which, however, is inherently limited by the temperature level of the refrigerant. Moreover, the propane refrigeration requires additional capital and operating cost and is recognized a safety concern in NGL plants. High ethane recovery of over 80% are rarely achievable with turbo expansion alone and thus propane refrigeration is required, adding complexity and safety hazards particularly in congested offshore and existing facilities environments. To circumvent at least some of the problems associated with relatively low efficiency and low recovery, Sorensen describes in U.S. Pat. No. 5,953,935 a plant configuration in which the absorber reflux is produced by compressing, cooling, and Joule-Thomson expansion of a slipstream of feed gas. Although Sorensen's configuration generally provides improved propane recovery, ethane recovery is typically limited to about 20% to 40%.
In yet other configurations, a turbo-expander is employed to provide cooling of the feed gas for high propane or ethane recovery. Exemplary configurations are described, for example, in U.S. Pat. No. 4,278,457, and U.S. Pat. No. 4,854,955, to Campbell et al., in U.S. Pat. No. 5,953,935 to McDermott et al., in U.S. Pat. No. 6,244,070 to Elliott et al., or in U.S. Pat. No. 5,890,377 to Foglietta. While such configurations may provide at least some advantages over other processes, they typically require modifications of turbo expanders and changes in operating conditions when the plants are changed from propane recovery mode to ethane recovery mode or vice versa, or when the feed gas composition changes over time. These known configurations are typically designed to operate within a narrow range of feed gas compositions and inlet pressures with the use of propane refrigeration. In most cases, high recoveries are also limited by CO2 freezing in the demethanizer, and propane recovery will drop in most cases when operating on ethane rejection mode.
To reduce refrigeration requirements, various configurations are known in which an additional lean reflux stream is routed to the demethanizer as described in WO04065868A2 and WO04080936A1 to Patel. Similarly, Pitman et al. describe in WO2007/001669A2 a plant in which a residue gas recycle stream is employed to control the temperature of the demethanizer for improved ethane recovery. Likewise, Mak et al. (WO2007/014069A2) teach use of a residue gas recycle stream and a lean cold feed gas to allow for increased ethane recovery. Alternatively, as described in U.S. Pat. No. 6,116,050 to Yao, a combined reflux with residue gas and deethanizer overhead is used in the demethanizer overhead, and a dual reflux scheme using residue gas recycle and deethanizer overhead is presented in WO2007/014209A2 to Schroeder et al. While such plants advantageously reduce energy consumption and increase C2 recovery to at least some extent, several disadvantages still remain. Most significantly, all or almost all of those configurations require a relatively fixed feed gas composition and typically lack of operational flexibility where a change in ethane recovery is required.
To circumvent at least some of the problems associated with the lack of flexibility of ethane recovery levels while maintaining high propane recovery, a twin reflux process described in U.S. Pat. No. 7,051,553 to Mak et al. has a configuration in which a first column receives two reflux streams: One reflux stream comprises a vapor portion of the NGL and the other reflux stream comprises a lean reflux provided by the overhead of the second distillation column. While such process is advantageous for varying ethane recovery levels to meet ethane market demand, it nevertheless requires external refrigeration and turbo expansion for feed gas cooling in order to maintain high recovery.
Thus, although various configurations and methods are known to recover natural gas liquids, all or almost all of them suffer from one or more disadvantages. Therefore, there is still a need to provide methods and configurations for improved natural gas liquids recovery.